Steel wires have heretofore been coated with various metals such as zinc, copper, tin and/or their alloy such as brass or bronze, in order to enhance their adhesion to rubber.
Typically, such coatings have been applied to the steel wire either electrolytically by electroplating in a suitable solution or by dipping or drawing through a molten metal. Generally, the electroplating method is preferred.
In a conventional electroplating process, the wire becomes the cathode, when charged with a negative polarity, as it is drawn through an aqueous solution, or electrolyte, in which is also immersed a metal anode, which is charged with a positive polarity. The electrolyte contains dissolved oppositely charged ions. As the negative polarity is applied to the wire cathode and a corresponding positive polarity applied to the anode, the cations move toward the cathode and the anions move toward the anode. Indeed, as the current passes between the cathode and the anode, the positive ions are attracted toward the negatively charged anode where their charge is neutralized and they are set free, leaving them deposited or plated, on the wire cathode. Similarly, the anions move to, and are discharged at, the anode.
Generally, the electrolyte is modified by containing a salt of the metal anode, thereby enhancing the deposition of the metal onto the wire cathode.
Indeed, the electrolyte, or aqueous plating bath, may consist of a number of optional components which may include (a) the said salt containing the metal ion, (b) an additional salt for the function of modifying the conductivity of the bath, (c) a compound to modify the anode corrosion and reduce its passivity, (d) an addition agent to effect the type of deposit produced and (e) a buffer to maintain or control the desired pH.
All of these materials, as well as their temperature and length of time of exposure to the wire cathode, have an effect on the rate of current consumed by the cathode and anode and, of particular importance, have an effect on the thickness and structure of metal deposition on the cathode wire. They may even effect the elasticity of the resultant plated wire composite.
All of these factors typically become of special importance when preparing a coated steel wire which has adhesion to rubber as its intended use.
Therefore, if an imbalance occurs between either the materials, the temperature, electrical current being applied, or even the time of exposure of the wire to the electrolyte, an off-specification coated wire can occur. Such off-specification can relate to coating thickness, the structure of the coating, the elasticity of the coating/wire composite, defective base wire itself, as well as actual portions of the wire which may have defective or no coating at all.
Unless an involved and generally prohibitively expensive recovery process is utilized, the off-specific tion wire has heretofore been usually scrapped.
In virtually any commercial electroplating process, disposal of off-specification electroplated products can be a problem. Recovery of the base metal has been reported by (a) chemical oxidation of the plated metal with acid to form a soluble salt, (b) electrolytic oxidation by reversing current flow in a conventional electroplating bath and (c) oxidation of the plated metal with a strong oxidizing agent (U.S. Pat. No. 2,937,940).
Chemical oxidative demetalizing is generally objectionable because of cost and necessity of additional equipment, chemicals and effluent treatment.
Electrolytic demetalization of brass electroplated steel wire, utilizing a reversal of current flow could be generally objectionable (a) in solutions containing cyanide because of the necessity of effluent treatment and (b) in solutions under acid conditions, the base steel wire would be degraded by pitting and the like. The problem is that most acidic electroplating baths would also attack the base material if it were positively charged.
Therefore, it is an object of this invention to provide a method of relatively efficiently recovering or deplating wire which has been electroplated.